1. Field of the Invention
The invention relates to an end port for a graphitizing furnace, especially for a Castner-type type or longitudinal graphitizing furnace, of refractory masonry and at least one electrode set into the masonry.
2. Description of the Prior Art
Furnaces for graphitizing carbon products consist in general of a rectangular furnace bed lined with granular refractory materials, ports at the end faces into which the graphite electrodes are set, and movable side walls. In the Acheson-type graphitizing furnace, the material to be graphitized is alternatingly stacked with layers of a granular resistance compound between the ports, and the stack is surrounded by a granular insulating compound. The heating to the graphitizing temperature of about 3000 K. is accomplished by resistance heating, the electric current being fed to the graphite electrodes in the ports via bus bars. With increasing temperature the charge material initially expands uniformly, but with the beginning emission of the sulfur contained in the carbon expands abruptly, and with an increasing degree of crystalline ordering or graphitizing, the volume of the charged material decreases. All volume changes are then taken up essentially by the granular resistance material, the packing density of which changes accordingly, so that no major forces act upon the graphite electrodes connected to the masonry of the ports in a positively force-transmitting manner. However, the formation of gaps between the masonry and the electrode does frequently occur, particularly because of the different coefficients of thermal expansion of these materials. Acheson-type graphitizing furnaces are rugged and prone to little disturbance; disadvantages are, among other things, the low energy efficiency and the low productivity per unit area. The graphitizing furnace initially proposed by Castner, which is often called a longitudinal graphitizing furnace, does not have these disadvantages. In a Castner furnace, the material to be graphitized, for instance in the form of cylindrical carbon bodies, is clamped between the graphite electrodes of the port without the interposition of layers of granular resistance material. At least one electrode is movable in the direction of the longitudinal axis of the furnace and, for obtaining a low contact resistance, the electrode is pressed against a strand formed by the carbon bodies to be graphitized braced against the counter-electrode. The length change of the strand in the graphitizing process varies; in the heating-up phase it is about +0.5 to 2% and in the cooling-down phase of the furnace, about -2 to 1.5% is taken up by a displacement of the electrodes in the opposite sense in the direction of the longitudinal oven axis. In order to have mobility of the electrode relative to the masonry of the oven head, it is necessary to have some clearance between the electrode and the masonry. Oxygen in the air penetrating into the gap between the electrode and the masonry reacts with carbon, and a wide gap could permit sufficient air to penetrate and cause the graphite electrode to burn up. It is also impossible fundamentally to adjust a constant small gap width over the entire temperature range because of the different coefficients of thermal expansion of the graphite electrode and the ceramic masonry. Attempts to design the gaps with stuffing gland-like seals and mineral fiber packings were not satisfactory since seals of this type are of only limited stability under the conditions of graphitizing. With increasing wear of the packing, the granular insulating material which shields the material to be graphitized against air, penetrates increasingly into the seal, whereby permanent gaps are formed and the inflow of air increases.